The present invention relates to an electric screw driver with an improved automatic turn-off device, particularly concerning an improved electric torsion-controlled screw driver with an improvement on the clutch mechanism and an automatic turn-off switch device which can control the electric screw driver effectively and conveniently.
FIG. 1a is a partial side sectional view of a prior electric screw driver showing a clutch mechanism 1 for driving the screw driver and a switch device 13. As the arrangement of a driving means for driving the screw driver, a handle switch for switching on said electric screw driver and a reducing gear assembly for transmitting rotating power to the clutch mechanism are conventional arrangements, they are not described in detail for the prior electric screw driver.
The clutch mechanism 1 includes a clutch housing 1a, an upper clutch member 10 which connects with the driving means through the reducing gear assembly and which is received in the clutch housing 1a, and a lower clutch member 11 received in the clutch housing 1a and connected the upper clutch member 10 by a rod member. The upper clutch member 10 further has a first triangular cam protrusion 100 at its bottom end and the lower clutch member 11 further includes a second triangular cam protrusion 110 at its upper end. A switch device 13 is provided at the head portion of the clutch housing and a supporting bowl 12 is received in the clutch housing 1a so that the upper edge of the supporting bowl 12 is engaged with the switch device 13. The supporting bowl 12 has a bottom opening so that a lower extending portion of the lower clutch member 11 passes through it and the upper portion of the lower clutch member is disposed against the bottom end of the supporting bowl 12.
A cylindrical cover member 1b with a bottom opening and an inner threaded surface is threaded on the outer threaded surface of the clutch housing 1a for covering the lower extending portion of the lower clutch member 11, wherein an inner flange portion is provided at the bottom opening of the cylindrical cover 1b. A retaining means 1c is provided at the bottom opening of the cylindrical cover member 1b and an elongated retaining rod member 1d which connects with the lower extending portion of the lower clutch member 11 is passed through the bottom opening and retained firmly by the retaining means 1c. A socket member 1e is connected with the outer end of the elongated rod member 1d for receiving a screw driver. (not shown in FIG. 1)
A torsion spring member 14 received in the cylindrical cover member 1b and sleeved on the lower clutch member, is disposed on the inner flange portion of the cylindrical cover member 1b for supporting and biasing the supporting bowl 12, whereby, the electric screw driver can tighten or loosen a screw of a workpiece if the electric screw driver is switched on through the handle switch as the driving means drives the clutch mechanism 1, the first triangular cam protrusion 100 of the upper clutch member 10 may engage with the second triangular cam protrusion 110 of the lower clutch member 11 (see FIG. 1b) and move the lower clutch member 11 down slightly when the lower clutch member 11 stops rotation if the screw is tightened tightly, so that the supporting bowl 12 is moved down and disengaged from the switch device 13, wherein the driving means stops as the switch device 13 is actuated by disengaging the supporting bowl 12 from it, even the handle switch is turned on. Then, after releasing the handle switch, the upper and lower clutch member 10, 11 return to their initial position due to the torsion of the torsion spring member 14.
The above-mentioned electric screw driver has the following drawbacks:
(1) The edge 101 of the first triangular cam protrusion 100 and the edge 111 of the second triangular cam protrusion 110 are easily damaged because they abrade each other each time the first cam protrusion 100 engages with and pushes down the second cam protrusion 110. This may decrease the precision of the torsion and cause unbalanced torsion.
(2) Generally, the initial driving force for unfastening a tightened screw is greater than the initial driving force for tightening a loosened screw. Although the switch device 13 and the clutch mechanism 1 to stop the driving means used in the prior electric screw driver are convenient in the screw-tightening operation, the switch device 13 may cause trouble in the screw-unfastening operation since a large initial driving force is necessary for unfastening a tightened screw. If the screw is tightened tightly, the lower clutch member 11 cannot rotate suddenly for the unfastening operation, when the upper clutch member 10 is driven by the driving means and is rotating so that the first cam protrusion 100 may engage and push down the second cam protrusion 110 of the lower clutch member 11. This may cause the switch device to turn off the driving means automatically. In this condition, the tightened screw cannot be unfastened by the electric screw driver easily, which is inconvenient for the user.
(3) The socket member 1e adapted to receive the screw driver is provided outside the cylindrical cover member 1b so that the screw driver portion extends far out and is difficult to control when handling the electric screwdriver.
(4) As the supporting bowl 12 and the clutch mechanism 1 are returned back to their initial position by the torsion spring member 14, the switch device 13 is always hit strongly by the upper edge of the supporting bowl 12 so that the switch device 13 may be easily damaged.
FIG. 1c is another prior electric screw driver with a slight improvement on the shape of the cam protrusions of the upper and lower clutch member. In this improvement, the upper clutch member 11 has a substantially triangular cam protrusion 103 with two identical arcs with symattrical curvatures (see FIG. 1d) instead of the first cam protrusion 100. The lower clutch member 11 has a ball shaped protrusion 15 instead of the second cam protrusion 110.
Although the above-mentioned improvement can reduce abrasion damage to the cam protrusion 103 and ball shaped protrusion 15, the other disadvantages such as hitting of the torsion spring member, and the long extension of the socket member for receiving the screw driver are not solved. In addition, the bottom summit portion of the substantially triangular cam protrusion 103 has a width `d` which may be damaged by frequent abrasion.